Hydroxyl-terminated derivatives of tetrabromophthalic anhydride are well known flame retardants. Thus tetrabromophthalic anhydride (TBPA) has a high bromine content (68.9%), by which it or its derivatives can be used to impart flame retardancy to a variety plastic systems, such as polyurethanes and especially polyurethane foams. Particular examples of commercial flame retardants based on hydroxyl-terminated derivatives of tetrabromophthalic anhydride are PHT-Diol™ supplied by Chemtura Corporation (a mixed ester of tetrabromophthalic anhydride with diethylene glycol and propylene oxide) and Saytex® RB-79 supplied by Albemarle Corporation (a mixed ester of tetrabromophthalic anhydride with diethylene glycol and propylene glycol).
Because diesterification of TBPA is difficult using conventional esterification techniques, the art has turned to a method which involves reacting TBPA with a polyhydric alcohol (e.g. glycols) to form a half-ester and, subsequently, reacting the half-ester with an alkylene oxide such as propylene oxide or ethylene oxide to form the diester. An example of such a process is disclosed in U.S. Pat. No. 3,455,886, in which an anhydride is first reacted with a polyhydric alcohol at a temperature of 80 to 150° C. and, only when substantially all of the anhydride has reacted with the alcohol, is an epoxide added to the reaction mixture and the reaction to form the final diester completed at a temperature of 60 to 160° C.
More recently a single step, batch wise process for producing tetrabromophthalic diester compositions has been proposed in U.S. Pat. No. 5,332,859. This process comprises (a) preparing a first batch by reacting, in an inert organic solvent, normally toluene, at a temperature up to about 150° C., tetrabromophthalic anhydride (TBPA), a C2 to C6 polyhydric aliphatic alcohol (PAA) and an alkylene oxide (AO) selected from the group consisting of ethylene oxide and propylene oxide, said reacting being in a PAA:AO:TBPA mole ratio of 1.6-1.9:1.3-1.5:1, so as to obtain a reacted mixture including the tetrabromophthalic diester composition and the organic solvent; (b) recovering the organic solvent from the reaction mixture by distillation; (c) analyzing the recovered organic solvent to determine the level of its AO content; and (d) preparing a second batch by repeating step (a) above wherein the organic solvent used includes the recovered organic solvent from the previous batch and wherein its determined AO level is accounted for in achieving the PAA:AO:TBPA mole ratio.
The diester product of the process of the '859 patent is a viscous liquid which becomes less viscous with increasing temperature. Typically, the product has a viscosity at 25° C. ranging from 80,000 to 150,000 cps, which imposes significant problems in handling the product during processing. As a result, formulators generally have to heat the product to move it through plant equipment. In addition, they add modifiers to reduce the viscosity of their formulated products. It would therefore be desirable to develop an improved single step process for producing tetrabromophthalic diester compositions in which the viscosity of the product can be reduced.
According to the present invention, it has now been found that, by conducting the single step process of the '859 patent in the absence of the toluene solvent, a lower viscosity product can be produced. Even in the absence of the toluene solvent, the remaining raw materials form a solution that is viscous but can be stirred. Therefore, the toluene is not necessary for agitation purposes. By removing the toluene from the process entirely, the process step to distill and recover toluene from the product is also eliminated. Not only does this avoid the costs associated with the toluene removal, but also having to remove toluene at the end of the reaction appears to be important to the viscosity of the product. In particular, it is believed that holding the reaction product at toluene strip temperatures for an extended period results in further reaction of any excess alkylene oxide and hence extension of the diol side chains. As the side chains get longer, increased chain entanglement is possible which results in higher viscosity at any given temperature.